We study the Majorana modes, both equilibrium and Floquet, which can appear at the edges of the Kitaev model on the honeycomb lattice. We first present the analytical solutions known for the equilibrium Majorana edge modes for both zigzag and armchair edges of a semi-infinite Kitaev model and chart the parameter regimes in which they appear. We then examine how edge modes can be generated if the Kitaev coupling on the bonds perpendicular to the edge is varied periodically in time as periodic $\delta$-function kicks. We derive a general condition for the appearance and disappearance of the Floquet edge modes as a function of the drive frequency for a generic $d$-dimensional integrable system. We confirm this general condition for the Kitaev model with a finite width by mapping it to a one-dimensional model. Our numerical and analytical study of this problem shows that Floquet Majorana modes can appear on some edges in the kicked system even when the corresponding equilibrium Hamiltonian has no Majorana mode solutions on those edges. We support our analytical studies by numerics for a finite sized system which show that periodic kicks can generate modes at the edges and the corners of the lattice.

We construct a family of simple fermionic projected entangled pair states (fPEPS) on the square lattice with bond dimension $D=3$ which are exactly hole-doped resonating valence bond (RVB) wave functions with short-range singlet bonds. Under doping the insulating RVB spin liquid evolves immediately into a superconductor with mixed $d+is$ pairing symmetry whose pair amplitude grows as the square root of the doping. The relative weight between $s$-wave and $d$-wave components can be controlled by a single variational parameter $c$. We optimize our ansatz with respect to $c$ for the frustrated $t\text{−}{J}_1\text{−}{J}_2$ model (including both nearest and next-nearest neighbor antiferromagnetic interactions $J_1$ and $J_2$, respectively) for $J_2\simeq J_1/2$ and obtain an energy very close to the infinite-PEPS state (using full update optimization and same bond dimension). The orbital symmetry of the optimized RVB superconductor has predominant $d$-wave character, although we argue a residual (complex $s$-wave) time reversal symmetry breaking component should always be present. Connections of the results to the physics of superconducting cuprates and pnictides are outlined.

We report measurements of the nonlinear susceptibility ${\chi }_3\left(T\right)$ in the metamagnetic heavy fermion (HF) compound ${\mathrm{UPt}}_3$. At high temperatures, ${\chi }_3\left(T\right)<0$ and is small. It turns positive for $T\le 35$ K, forms a peak at $T\cong 10$ K, and then decreases to zero with further decreasing temperature. The peak in $\chi {}_3$ occurs at a temperature $T_3$, which is roughly half of $T_1$, the temperature of the maximum in the linear susceptibility. We present results on ${\mathrm{URu}}_2{\mathrm{Si}}_2$ and ${\mathrm{UPd}}_2{\mathrm{Al}}_3$ to show that this feature is common to other HF materials. A two-level model to describe the metamagnetic transition, with separation between the levels being the only energy scale, captures all experimentally observed features.

In the classical analysis of charge transport in solids by Lampert in 1956 an Ohmic current is attributed to the presence of a background carrier density due to (unintentional) doping. We demonstrate that Ohmic currents are also observed in undoped semiconductors as a result of diffusion of charge carriers from the contacts into the semiconductor. This Ohmic diffusion current shows an enhanced thickness scaling and is governed by the charge-carrier mobility. Specific for organic semiconductors the charge-carrier density dependence of the mobility at zero electric field can be accurately determined from the Ohmic diffusion current.

We report on a theoretical and experimental study of time-optimal construction of arbitrary single-qubit rotations under a single strong driving field of finite amplitude. Using radiation-dressed states of nitrogen vacancy centers in diamond we realize a strongly driven two-level system, with driving frequencies four times larger than its precession frequency. We implement time-optimal universal rotations on this system, characterize their performance using quantum process tomography, and demonstrate a dual-axis multiple-pulse control sequence where the qubit is rotated on time scales faster than its precession period. Our results pave the way for applying fast qubit control and high-density pulse schemes in the fields of quantum information processing and quantum metrology.

We study the edge magnetization and the local density of states of chiral graphene nanoribbons using a $\pi \text{-orbital}$ Hubbard model in the mean-field approximation. We show that the inclusion of a realistic next-nearest hopping term in the tight-binding Hamiltonian changes the graphene nanoribbon band structure significantly and affects its magnetic properties. We study the behavior of the edge magnetization upon departing from half-filling as a function of the nanoribbon chirality and width. We find that the edge magnetization depends very weakly on the nanoribbon width, regardless of chirality as long as the ribbon is sufficiently wide. We compare our results to recent scanning tunneling microscopy experiments reporting signatures of magnetic ordering in chiral nanoribbons and provide an interpretation for the observed peaks in the local density of states, that does not depend on the antiferromagnetic interedge interaction.

Recently, the topological classification of electronic states has been extended to a new class of matter known as topological crystalline insulators. Similar to topological insulators, topological crystalline insulators also have spin-momentum locked surface states, but they only exist on specific crystal planes that are protected by crystal reflection symmetry. Here, we report an ultralow temperature scanning tunneling microscopy and spectroscopy study on topological crystalline insulator SnTe nanoplates grown by molecular beam epitaxy. We observed quasiparticle interference patterns on the SnTe (001) surface that can be interpreted in terms of electron scattering from the four Fermi pockets of the topological crystalline insulator surface states in the first surface Brillouin zone. A quantitative analysis of the energy dispersion of the quasiparticle interference intensity shows two high energy features related to the crossing point beyond the Lifshitz transition when the two neighboring low energy surface bands near the $\overline{\mathrm{X}}$ point merge. A comparison between the experimental and computed quasiparticle interference patterns reveals possible spin texture of the surface states.

Hydrogen-vacancy complexes can form in a material due to the exothermic binding of hydrogen atoms to vacancy sites. We explore the structure and electronic properties of hydrogen-vacancy complexes in δ-Pu using a density functional theory supercell approach, with up to eight hydrogen atoms stored in the vacancy site. We find that the hydrogen atoms bind to the inner edge of the vacancy site, preferring pseudo-octahedral configurations that optimize the Pu-H bond length. Hydrogen binding to the vacancy site remains exothermic, with binding energies around −0.4 eV/H atom. A statistical mechanics analysis is derived and applied to reveal the range of hydrogen chemical potentials that would lead to hydrogen-vacancy complex formation. We find that these chemical potentials are higher than those required to form the hydride phase, indicating that hydriding should occur before any appreciable concentration of vacancy-hydrogen complexes is realized. Some remarks are made comparing this theoretical finding to the experimental work on this topic, with suggestions given for future work that may help reconcile some apparent contradictions.

The negative thermal expansion (NTE) mechanism in ${\mathrm{Cu}}_2\mathrm{O}$ has been characterized via mapping of different ${\mathrm{Cu}}_2\mathrm{O}$ structural flexibility models onto phonons obtained using ab initio lattice dynamics. Low-frequency acoustic modes that are responsible for the NTE in this material correspond to vibrations of rigid O-Cu-O rods. There is also some small contribution from higher-frequency optic modes that correspond to rotations of rigid and near-rigid ${\mathrm{OCu}}_4$ tetrahedra as well as of near-rigid O-Cu-O rods. The primary NTE mode also drives a ferroelastic phase transition at high pressure; our calculations predict this to be an orthorhombic structure with space group $Pnnn$.

We propose to study the absorption of the ac magnetic field polarized parallel to the dc field (longitudinal spin pumping), applied to a one-dimensional (1D) topological superconductor. Such a longitudinal pumping in 1D topological superconductors does not produce a parametric instability due to the fermionic character of eigenstates. It is shown that Majorana edge states can manifest themselves in the absorption of the longitudinal spin pumping.

A Monte-Carlo algorithm for discrete statistical models that combines the full power of the belief-propagation algorithm with the advantages of a heat-bath approach fulfilling the detailed balance is presented. First we extract randomly a subtree inside the interaction graph of the system. Second, given the boundary conditions, belief propagation is used as a perfect sampler to generate a configuration on the tree, and finally, the procedure is iterated. This approach is best adapted for locally treelike graphs and we therefore tested it on random graphs for hard models such as spin glasses, demonstrating its state-of-the art status in those cases.

We have developed a fully microscopic theory of magnetic properties of the prototype molecular magnet ${\mathrm{Mn}}_{12}$. First, the intramolecular magnetic properties have been studied by means of first-principles density functional based methods, with local correlation effects being taken into account within the local density approximation plus $U$ $(\text{LDA}+U)$ approach. Using the magnetic force theorem, we have calculated the interatomic isotropic and anisotropic exchange interactions and full tensors of single-ion anisotropy for each Mn ion. Dzyaloshinskii-Moriya (DM) interaction parameters turned out to be unusually large, reflecting a low symmetry of magnetic pairs in molecules, in comparison with bulk crystals. Based on these results we predict a distortion of ferrimagnetic ordering due to DM interactions. Further, we use an exact diagonalization approach allowing one to work with as large a Hilbert space dimension as ${\mathrm{10}}^8$ without any particular symmetry (the case of the constructed magnetic model). Based on the computational results for the excitation spectrum, we propose a distinct interpretation of the experimental inelastic neutron scattering spectra.

We study normal state electrical, thermoelectrical, and thermal transport in polycrystalline ${\mathrm{BiS}}_2$-based compounds, which become superconducting by F doping on the O site. In particular, we explore undoped ${\mathrm{LaOBiS}}_2$ and doped ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ samples, prepared either with or without high-pressure annealing, in order to evidence the roles of doping and preparation conditions. The high-pressure annealed sample exhibits room temperature values of resistivity ρ around 5 mΩcm, Seebeck coefficient $S$ around $-20\mu V/K$, and thermal conductivity κ around 1.5 W/Km, while the Hall resistance $R_H$ is negative at all temperatures and its value is $-10{}^{-8} m{}^3/\mathrm{C}$ at low temperature. The sample prepared at ambient pressure exhibits $R_H$ positive in sign and five times larger in magnitude, and $S$ negative in sign and slightly smaller in magnitude. These results reveal a complex multiband evolution brought about by high-pressure annealing. In particular, the sign inversion and magnitude suppression of $R_H$, indicating increased electron-type carrier density in the high-pressure sample, may be closely related to previous findings about change in lattice parameters and enhancement of superconducting $T_c$ by high-pressure annealing. As for the undoped sample, it exhibits 10 times larger resistivity, 10 times larger $\left|S\right|$, and 10 times larger $|R_H|$ than its doped counterpart, consistent with its insulating nature. Our results point out the dramatic effect of preparation conditions in affecting charge carrier density as well as structural, band, and electronic parameters in these systems.

We investigate the superconducting pairing instabilities of eight-band models for the iron arsenides. Using a functional renormalization group treatment, we determine how the critical energy scale for superconductivity depends on the electronic band structure. Most importantly, if we vary the parameters from values corresponding to LaFeAsO to SmFeAsO, the pairing scale is strongly enhanced, in accordance with the experimental observation. We analyze the reasons for this trend and compare the results of the eight-band approach to those found using five-band models.

We study the effects of electrostatic gating on the current-induced phenomena in ultrathin ferromagnet/heavy metal heterostructures. We utilize heterodyne detection and analysis of symmetry with respect to the direction of the magnetic field to separate electric field contributions to the magnetic anisotropy, current-induced fieldlike torque, and damping torque. Analysis of the electric field effects allows us to estimate the Rashba and the spin Hall contributions to the current-induced phenomena. Electrostatic gating can provide insight into the spin-orbit phenomena, and enable new functionalities in spintronic devices.

Extended x-ray absorption fine-structure studies have been performed at the Zn $K$ and Cd $K$ edges for a series of solid solutions of wurtzite ${\mathrm{Zn}}_{1-x}{\mathrm{Cd}}_x$S samples with $x$ = 0.0, 0.1, 0.25, 0.5, 0.75, and 1.0, where the lattice parameter as a function of $x$ evolves according to the well-known Vegard's law. In conjunction with extensive, large-scale first-principles electronic structure calculations with full geometry optimizations, these results establish that the percentage variation in the nearest-neighbor bond distances are lower by nearly an order of magnitude compared to what would be expected on the basis of lattice parameter variation, seriously undermining the chemical pressure concept. With experimental results that allow us to probe up to the third coordination shell distances, we provide a direct description of how the local structure, apparently inconsistent with the global structure, evolves very rapidly with interatomic distances to become consistent with it. We show that the basic features of this structural evolution with the composition can be visualized with nearly invariant ${\mathrm{Zn-S}}_4$ and ${\mathrm{Cd-S}}_4$ tetrahedral units retaining their structural integrity, while the tilts between these tetrahedral building blocks change with composition to conform to the changing lattice parameters according to the Vegard's law within a relatively short length scale. These results underline the limits of applicability of the chemical pressure concept that has been a favored tool of experimentalists to control physical properties of a large variety of condensed matter systems.

The filled skutterudite ${\mathrm{LaFe}}_4{\mathrm{Sb}}_{12}$ shows greatly reduced thermal conductivity compared to that of the related unfilled compound ${\mathrm{CoSb}}_3$, although the microscopic reasons for this are unclear. We calculate harmonic and anharmonic force constants for the interaction of the La filler atom with the framework atoms. We find that force constants show a general trend of decaying rapidly with distance and are very small for the interaction of the La with its next-nearest-neighbor Sb and nearest-neighbor La. However, a few rather long-range interactions, such as with the next-nearest-neighbor La and with the third neighbor Sb, are surprisingly strong, although still small. We test the central-force approximation and find significant deviations from it. Using our force constants we calculate a bare La mode Gruneisen parameter and find a value of 3–4, substantially higher than values associated with cage atom anharmonicity, i.e., a value of about 1 for ${\mathrm{CoSb}}_3$ but much smaller than a previous estimate [Bernstein et al., Phys. Rev. B 81, 134301 (2010)]. This latter difference is primarily due to the previously used overestimate of the La-Fe cubic force constants. We also find a substantial negative contribution to this bare La Gruneisen parameter from the aforementioned third-neighbor La-Sb interaction. Our results underscore the need for rather long-range interactions in describing the role of anharmonicity on the dynamics in this material.

${\mathrm{Cr}}_2\mathrm{B}$ displays temperature-independent paramagnetism. We induce ferromagnetism by replacing less than $3\%$ of the Cr atoms by Fe. By the lowest Fe doping level made, Curie-Weiss behavior is observed; ${\Theta }_{\mathrm{CW}}$ changes from $-20$K for $0.5\%$ Fe-doped ${\mathrm{Cr}}_2\mathrm{B}$ to positive values of about 50 K by $5\%$ Fe doping. The ferromagnetic $\mathit{T}{}_C$ is 8 K for $2.5\%$ Fe doping and increases linearly to 46 K by $5\%$ doping; we infer that a quantum phase transition occurs near the $2.0\%$ Fe level. Magnetic fluctuations at the intermediate doping levels are reflected in the linear resistance and an anomalous heat capacity at low temperatures. Imaging and chemical analysis down to the atomic scale show that the Fe dopant is randomly distributed.

We study the nonequilibrium dynamics of quenching through a quantum critical point in topological systems, focusing on one of their defining features: ground-state degeneracies and associated topological sectors. We present the notion of “topological blocking,” experienced by the dynamics due to a mismatch in degeneracies between two phases, and we argue that the dynamic evolution of the quench depends strongly on the topological sector being probed. We demonstrate this interplay between quench and topology in models stemming from two extensively studied systems, the transverse Ising chain and the Kitaev honeycomb model. Through nonlocal maps of each of these systems, we effectively study spinless fermionic $p$-wave paired topological superconductors. Confining the systems to ring and toroidal geometries, respectively, enables us to cleanly address degeneracies, subtle issues of fermion occupation and parity, and mismatches between topological sectors. We show that various features of the quench, which are related to Kibble-Zurek physics, are sensitive to the topological sector being probed, in particular, the overlap between the time-evolved initial ground state and an appropriate low-energy state of the final Hamiltonian. While most of our study is confined to translationally invariant systems, where momentum is a convenient quantum number, we briefly consider the effect of disorder and illustrate how this can influence the quench in a qualitatively different way depending on the topological sector considered.

A layered perovskite ${\mathrm{LaSrVO}}_4$ was studied by neutron diffraction, pair distribution function measurement using synchrotron x-ray, susceptibility, and specific heat measurements, and first-principles calculation. The results show (i) a weak structural distortion around 100 K with the existence of orbital fluctuations both above and below it; (ii) the absence of the long range magnetic ordering down to 0.35 K but the appearance of a short range magnetic ordering around 11 K with a $T^2$ behavior of the specific heat below it. Meanwhile, the calculation based on the density functional theory predicts a magnetic ordered ground state. All facts indicate a melting of the magnetic ordering due to the orbital fluctuations in ${\mathrm{LaSrVO}}_4$, which makes it a rare candidate for the spin-orbital liquid state related to $t_{2g}$ orbitals.