Search

Recent experiments on certain Fe-based superconductors have hinted at a role for paired electrons in "incipient" bands that are close to, but do not cross, the Fermi level. Related theoretical works disagree on whether or not strong-coupling superconductivity is required to explain such effects, and whether a critical interaction strength exists. In this work, we consider various versions of the model problem of pairing of electrons in the presence of an incipient band, within a simple multiband weak-coupling BCS approximation. We categorize the problem into two cases: case (i), where superconductivity arises from the "incipient band pairing" alone, and case (ii), where it is induced on an incipient band by pairing due to Fermi-surface-based interactions. Negative conclusions regarding the importance of incipient bands have been drawn so far largely based on case (i), but we show explicitly that models under case (ii) are qualitatively different, and can explain the nonexponential suppression of Tc, as well as robust large gaps on an incipient band. In the latter situation, large gaps on the incipient band do not require a critical interaction strength. We also model the interplay between phonon and spin fluctuation driven superconductivity and describe situations in which they can enhance each other rather than compete. Finally, we discuss the effect of the dimensionality of the incipient band on our results. We argue that pairing on incipient bands may be significant and important in several Fe-based materials, including LiFeAs, FeSe intercalates, and FeSe monolayers on strontium titanate, and indeed may contribute to high critical temperatures in some cases. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of the thermodynamic and electronic properties of intrinsic point defects in the solar energy conversion materials Cu2ZnSnSe4 and CuInSe2 based on the screened-exchange hybrid density functional theory. A comparison between the defect transition levels for Cu2ZnSnSe4 and CuInSe2 reveals that in Cu2ZnSnSe4, the SnCu and SnZn antisite defects can be recombination centers with defect states close to midgap, while the InCu antisite defect has a shallow defect level in CuInSe2. The resultant higher Shockley-Read-Hall recombination rate in Cu2ZnSnSe4 reduces the steady-state concentration of minority carriers and quasi-Fermi level separation under illumination. This may explain the origin of the low open-circuit voltage values for Cu2ZnSnSe4 solar cells compared to CuInSe2 solar cells. [ABSTRACT FROM AUTHOR]

Motivated by the studies of the superconducting pairing states in the iron-based superconductors, we analyze the effects of Brillouin zone folding procedure from a space-group symmetry perspective for a general class of materials with the P4/nmm space group. The Brillouin zone folding amounts to working with an effective 1-Fe unit cell, instead of the crystallographic 2-Fe unit cell. We show that the folding procedure can be justified by the validity of a glide reflection symmetry throughout the crystallographic Brillouin zone and by the existence of a minimal double degeneracy along the edges of the latter. We also demonstrate how the folding procedure fails when a local spin-orbit coupling is included although the latter does not break any of the space-group symmetries of the bare Hamiltonian. In light of these general symmetry considerations, we further discuss the implications of the glide reflection symmetry for the superconducting pairing in an effective multiorbital t-J1-J² model. We find that, for spin-singlet pairing states, the P4/nmm space-group symmetry allows only even parity under the glide reflection and zero total momentum. [ABSTRACT FROM AUTHOR]

Using density functional calculations, we study a set of candidate defects for Fermi-level pinning at GaAs/oxide interfaces. The set of considered defects comprises both bulklike and interfacial defects, including As antisites, Ga and As dangling bonds, the As-As dimer/dangling bond defect, and several defect complexes. The defects are generated within atomistic model structures representing the GaAs/Al2O3 interface. Formation energies of bulklike defects are obtained and compared with those of corresponding bulk defects, while interfacial defects are studied through their relative defect energies. Finite-size corrections to the defect energies are applied through a scheme that accounts for the interfacial geometry of our models. Defect levels are defined as thermodynamic transition levels between different charge states and are calculated for all considered defects. Through an alignment procedure based on hybrid functional calculations, the defect levels are then positioned within the calculated band gap of GaAs that reproduces the experimental one, thereby enabling direct comparisons with the experimental density of defect states. Our study shows that several As-related defects show a similar amphoteric bistability between an As-As dimer state and a configuration with two doubly occupied As dangling bonds. The associated charge transition levels generally he in the midgap region, in accord with experimental observations. This mechanism is proposed as the origin of the observed Fermi-level pinning at GaAs/oxide interfaces. [ABSTRACT FROM AUTHOR]

Energy transfer from electrons to phonons is an important consideration in any Weyl- or Dirac-semimetal-based application. In this work, we analytically calculate the cooling power of acoustic phonons, i.e., the energy relaxation rate of electrons which are interacting with acoustic phonons, for Weyl and Dirac semimetals in a variety of different situations. For cold Weyl or Dirac semimetals with the Fermi energy at the nodal points, we find that the electronic temperature Te decays in time as a power law. In the heavily doped regime, Te decays linearly in time far from equilibrium. In a heavily doped system with short-range disorder we predict that the cooling power of acoustic phonons is drastically increased because of an enhanced energy transfer between electrons and phonons. When an external magnetic field is applied to an undoped system, the cooling power is linear in magnetic field strength and Te has square-root decay in time, independent of magnetic-field strength over a range of values. [ABSTRACT FROM AUTHOR]

We report the change of electronic structure associated with the hidden-order (HO) transition in URu2Si2 through Compton scattering experiment on the (001) plane of the single crystal. The two-dimensional electron occupation number densities (EONDs) obtained at 14 and 20 K, which are the projection of Fermi volume onto the kx-ky plane of the first Brillouin zone, clearly reflect a whole image of electronic structure in the HO and paramagnetic (PM) phases, respectively. The change in electronic structure is well described by theoretical EONDs derived from the result of a band calculation, where U 5f electrons are treated as itinerant ones. We also evaluate the electron (hole) numbers in the HO and PM phases and, therefore, its change on the HO transition. The HO transition is deeply involved with a significant change in 5f electrons from partially localized to itinerant states through hybridization with conduction electrons, resulting in the reconstmction of Fermi-surface structure. [ABSTRACT FROM AUTHOR]

We report a high-resolution angle-resolved photoemission spectroscopy study of the evolution of the electronic structure of FeSe1-xSx single crystals. Isovalent S substitution onto the Se site constitutes a chemical pressure which subtly modifies the electronic structure of FeSe at high temperatures and induces a suppression of the tetragonal-symmetry-breaking structural transition temperature from 87 to 58 K for x = 0.15. With increasing S substitution, we find smaller splitting between bands with dyz and dxz orbital character and weaker anisotropic distortions of the low-temperature Fermi surfaces. These effects evolve systematically as a function of both S substitution and temperature, providing strong evidence that an orbital ordering is the underlying order parameter of the structural transition in FeSe1-xSx. Finally, we detect the small inner hole pocket for x = 0.12, which is pushed below the Fermi level in the orbitally ordered low-temperature Fermi surface of FeSe. [ABSTRACT FROM AUTHOR]

We have investigated thermoelectric properties of a three-dimensional Rashba system BiTel. Magnetic-field dependences of the Seebeck effect and Nernst effect show qualitative changes with the Fermi level passing through the bulk Dirac point, indicating that thermoelectric effects can be a good experimental probe for the Fermi surface topology. The quantum oscillations are observed in the thermoelectric effect of BiTel under a magnetic field, which are also dependent on the Fermi-level positions and consistent with the energy derivative of the three-dimensional density of states in the Rashba system. [ABSTRACT FROM AUTHOR]

In a previous work [Phys. Rev. B 84, 085120 (2011)], we presented the orbital-separation approach for simulating metal/insulator/metal capacitors under bias voltage within the Kohn-Sham framework of density functional theory. A limitation mentioned in that work was that it is not straightforward to calculate the amount of free charge transferred from one electrode to the other, and thus the application of the method was limited to the closed-circuit (constant-voltage) condition. Here, we show that it is actually possible in practice to calculate the free charge by examining the change in the occupation of the Kohn-Sham eigenstates near the Fermi level. Thus it is also possible to perform open-circuit (constant-charge) simulations, and we demonstrate this on a metal/ferroelectric/metal capacitor. [ABSTRACT FROM AUTHOR]

We develop a theory for electron-electron interaction-induced many-body effects in three-dimensional Weyl or Dirac semimetals, including interaction corrections to the polarizability, electron self-energy, and vertex function, up to second order in the effective fine-structure constant of the Dirac material. These results are used to derive the higher-order ultraviolet renormalization of the Fermi velocity, effective coupling, and quasiparticle residue, revealing that the corrections to the renormalization group flows of both the velocity and coupling counteract the leading-order tendencies of velocity enhancement and coupling suppression at low energies. This in turn leads to the emergence of a critical coupling above which the interaction strength grows with decreasing energy scale. In addition, we identify a range of coupling strengths below the critical point in which the Fermi velocity varies nonmonotonically as the low-energy, noninteracting fixed point is approached. Furthermore, we find that while the higher-order correction to the flow of the coupling is generally small compared to the leading order, the corresponding correction to the velocity flow carries an additional factor of the Dirac cone flavor number (the multiplicity of electron species, e.g. ground-state valley degeneracy arising from the band structure) relative to the leading-order result. Thus, for materials with a larger multiplicity, the regime of velocity nonmonotonicity is reached for modest values of the coupling strength. This is in stark contrast to an approach based on a large-TV expansion or the random phase approximation (RPA), where higher-order corrections are strongly suppressed for larger values of the Dirac cone multiplicity. This suggests that perturbation theory in the coupling constant (i.e., the loop expansion) and the RPA/large-N expansion are complementary in the sense that they are applicable in different parameter regimes of the theory. We show how our results for the ultraviolet renormalization of quasiparticle properties can be tested experimentally through measurements of quantities such as the optical conductivity or dielectric function (with carrier density or temperature acting as the scale being varied to induce the running coupling). Although experiments typically access the finite-density regime, we show that our zero-density results still capture clear many-body signatures that should be visible at higher temperatures even in real systems with disorder and finite doping. [ABSTRACT FROM AUTHOR]

By means of ab initio calculations we study the effect of O doping of Au chains containing a nanocontact represented by a Ni atom as a magnetic impurity. In contrast to pure Au chains, we find that with a minimum O doping the 5dxz,yz states of Au are pushed up, crossing the Fermi level. We also find that for certain O configurations, the Ni atom has two holes in the degenerate 3dxz,yz orbitals, forming a spin S=1 due to a large Hund interaction. The coupling between the 5dxz,yz Au bands and the 3dxz,yz of Ni states leads to a possible realization of a two-channel S=1 Kondo effect. While this kind of Kondo effect is commonly found in bulk systems, it is rarely observed in low dimensions. The estimated Kondo scale of the system lies within the present achievable experimental resolution in transport measurements. Another possible scenario for certain atomic configurations is that one of the holes resides in a 3dz2 orbital, leading to a two-stage Kondo effect, the second one with SU(4) symmetry. [ABSTRACT FROM AUTHOR]

Combining photoemission spectroscopy, Raman spectroscopy, and first-principles calculations, we characterize superconducting TlNi2Se2 as a material with weak electronic Coulomb correlations leading to a bandwidth renormalization of 1.4. We identify a camelback-shaped band, whose energetic position strongly depends on the selenium height. While this feature is universal in transition metal pnictides, in TlNi2Se2 it lies in the immediate vicinity of the Fermi level, giving rise to a pronounced van Hove singularity. The resulting heavy band mass resolves the apparent puzzle of a large normal-state Sommerfeld coefficient [H. Wang et al., Phys. Rev. Lett 111, 207001 (2013)] in this weakly correlated compound. [ABSTRACT FROM AUTHOR]

Nuclear magnetic resonance (NMR) experiments on single crystals of HgBa2CuO4+d are presented that identify two distinct temperature-dependent spin susceptibilities: One is due to a spin component that is temperature-dependent above the critical temperature for superconductivity (Tc) and reflects pseudogap behavior; the other is Fermi-liquid-like in that it is temperature independent above Tc and vanishes rapidly below Tc. In addition, we demonstrate the existence of a third spin susceptibility: It is temperature independent at higher temperatures, vanishes at lower temperatures (below T0≠Tc), and changes sign near optimal doping. This susceptibility either arises from the coupling between the two spin components, or it could be given by a distinct third spin component. Recent susceptibility data on single crystals support its presence in most cuprates. [ABSTRACT FROM AUTHOR]

We study with angle-resolved photoelectron spectroscopy the evolution of the electronic structure of Sr2IrO4, when holes or electrons are introduced, through Rh or La substitutions. At low dopings, the added carriers occupy the first available states, at the bottom or top of the gap, revealing an asymmetric gap of 0.7 eV, in good agreement with scanning tunnel microscope measurements. At further doping, we observe a reduction of the gap and a transfer of spectral weight across the gap, although the quasiparticle weight remains very small. We discuss the origin of the in-gap spectral weight as a local distribution of gap values. [ABSTRACT FROM AUTHOR]

In a combined experimental and theoretical study, we investigate the properties of Sr2Ir1-xRhxO4. From the branching ratios of the L-edge isotropic x-ray absorption spectra, we determine that the spin-orbit coupling is remarkably independent of x for both iridium and rhodium sites. DFT+U calculations show that the doping is close to isoelectronic and introduces impurity bands of predominantly rhodium character close to the lower Hubbard band. Overlap of these two bands leads to metallic behavior. Since the low-energy states for x < 0.5 have predominantly jeff = ½ character, we suggest that the electronic properties of this material can be described by an inhomogeneous Hubbard model, where the on-site energies change due to local variations in the spin-orbit interaction strength combined with additional changes in binding energy. [ABSTRACT FROM AUTHOR]

We study alkali-metal adsorption on supported graphene by means of density-functional-theory calculations that include dispersion corrections. Graphene supported by the Au/Ni(111) surface is an important system for fundamental studies because this surface allows one to support graphene, preserving the electronic properties of freestanding graphene. We investigate the binding energetics as well as the structural and electronic properties of Li, Na, K, and Rb atoms adsorbed or intercalated at the graphene/Au/Ni(111) interfaces and compare the results to those obtained on freestanding graphene. Both the adsorption and intercalation of the alkali atoms induce electron doping to the graphene π bands that display a quasirigid-energy shift. Electron doping by alkali atoms preserves the Dirac cone of graphene, which shifts downwards due to the negative polarity of the doping. The metallic or ionic character of the alkali dopant is controlled by its position relative to graphene. All of the investigated alkali atoms display an energetic preference to intercalate between graphene and the Au/Ni support. These results shed light on the relationship between graphene electron doping, the alkali-atom adsorption site, and the charge transfers at the graphene/support interface. [ABSTRACT FROM AUTHOR]

In this work we introduce a low-energy Hamiltonian for single-layer and bilayer black phosphorus that describes the electronic states at the vicinity of the Γ point. The model is based on a recently proposed tight-binding description for electron and hole bands close to the Fermi level. We calculate expressions for the Landau-level spectrum as function of magnetic field, and in the case of bilayer black phosphorus we investigate the effect of an external bias on the electronic band gap. The results showcase the highly anisotropic character of black phosphorus, and in particular for bilayer BP, the presence of bias allows for a field-induced semiconductor-metal transition. [ABSTRACT FROM AUTHOR]

We consider integer quantum Hall states and calculate the bulk entanglement spectrum by formulating the correlation matrix in the guiding center representation. Our analytical approach is based on the strategy of redefining the inner product of states in the Hilbert space, via a projection operator, to take care of the restriction imposed by the (rectangle-pixeled) checkerboard partition. The resultant correlation matrix contains various couplings between states of different guiding centers parameterized by the magnetic length and the pixel size. Given a fixed magnetic field, we find various patterns of band crossings by tuning the pixel size (quantified by the flux Φ threading each pixel) and by changing the filling factor v ∊ N (determined by the Fermi level). When v = 1 and Φ = 2π, or v = 2 and Φ = π, one Dirac band crossing is found. For v = 1 and Φ = π, the band crossings are in the form of a nodal line, enclosing the Brillouin zone. As for v = 2 and Φ = 2π, the doubled Dirac point, or the quadratic point, is seen. Additionally, we infer that the quadratic point is protected by the C4 symmetry of the pixel since it evolves into two separate Dirac points when the symmetry is lowered to C2. We also identify the emerging symmetries responsible for the symmetric bulk entanglement spectra, which are absent in the underlying quantum Hall states. [ABSTRACT FROM AUTHOR]

We analyzed the valence band (VB) of the 90 K high-temperature superconductor YBa2Cu3O7-δ by photoelectron spectroscopy under standing-wave excitation employing hard x rays. Precisely positioning the standing-wave intensity in the unit cell allows selectively probing the VB yield from the CuO chains and CuO2 planes, respectively. Both contribute strongly over the whole VB but the spectral weight of the planes is significantly higher than the chains within about 2 eV from the Fermi level. In the x-ray regime, the major contribution to the VB emission is coming from Cu 3d. [ABSTRACT FROM AUTHOR]

We propose a method to compute the transmission through correlated heterostructures by combining density functional and many-body dynamical mean field theories. The heart of this combination consists in porting the many-body self-energy from an all electron basis into a pseudopotential localized atomic basis set. Using this combination we study the effects of local electronic interactions and finite temperatures on the transmission across the Cu4CoCu4 metallic heterostructure. It is shown that as the electronic correlations are taken into account via a local but dynamic self-energy, the total transmission at the Fermi level gets reduced (predominantly in the minority-spin channel), whereby the spin polarization of the transmission increases. The latter is due to a more significant d-electron contribution, as compared to the noncorrelated case in which the transport is dominated by s and p electrons. [ABSTRACT FROM AUTHOR]

In two-dimensional topological insulators, such as inverted HgTe/CdTe quantum wells, helical quantum spin Hall (QSH) states persist even at finite magnetic fields below a critical magnetic field Bc, above which only quantum Hall (QH) states can be found. Using linear-response theory, we theoretically investigate the magneto-optical properties of inverted HgTe/CdTe quantum wells, both for infinite two-dimensional and finite-strip geometries and for possible signatures of the transition between the QSH and QH regimes. In the absorption spectrum, several peaks arise due to nonequidistant Landau levels in both regimes. However, in the QSH regime, we find an additional absorption peak at low energies in the finite-strip geometry. This peak arises due to the presence of edge states in this geometry and persists for any Fermi level in the QSH regime, while in the QH regime the peak vanishes if the Fermi level is situated in the bulk gap. Thus, by sweeping the gate voltage, it is possible to experimentally distinguish between the QSH and QH regimes due to this signature. Moreover, we investigate the effect of spin-orbit coupling and finite temperature on this measurement scheme. [ABSTRACT FROM AUTHOR]

We present a detailed investigation of spectroscopic features located at the central metal ion of MnPc (where Pc represents phthalocyanine) on Ag(001) by means of scanning tunneling spectroscopy (STS) and first-principles theory. STS data taken close to the Fermi level reveal an asymmetric feature that cannot be fitted with a single Fano function representing a one-channel Kondo effect. Instead, our data indicate the existence of a second superimposed feature. Two potential physical origins, a second Kondo channel related to the dxz/yz orbitals, and a spectral feature of the dz2 orbital itself, are discussed. A systematic experimental and theoretical comparison of MnPc with CoPc and FePc indicates that the second feature observed on MnPc is caused by the dz2 orbital. This conclusion is corroborated by STM-induced dehydrogenation experiments on FePc and MnPc which in both cases result in a gradual shift towards more positive binding energies and a narrowing of the Kondo resonance. Theoretical analysis reveals that the latter is caused by the reduced hybridization between the d orbital and the substrate. Spatially resolved differential conductivity maps taken close to the respective peak positions show that the intensity of both features is highest over the central Mn ion, thereby providing further evidence against a second Kondo channel originating from the dxz/yz orbital of the central Mn ion. [ABSTRACT FROM AUTHOR]

We use angle-resolved photoemission spectroscopy to extract the band dispersion and the Fermi surface of BaCu2As2 and α-BaCu2Sb2. While the Cu 3d bands in both materials are located around 3.5 eV below the Fermi level, the low-energy photoemission intensity mainly comes from As 4p states, suggesting a completely filled Cu 3d shell. The splitting of the As 3d core levels and the lack of pronounced three-dimensionality in the measured band structure of BaCu2As2 indicate a surface state likely induced by the cleavage of this material in the collapsed tetragonal phase, which is consistent with our observation of a Cu+1 oxidation state. However, the observation of Cu states at similar energy in α-BaCu2Sb2 without the pnictide-pnictide interlayer bonding characteristic of the collapsed tetragonal phase suggests that the short interlayer distance in BaCu2As2 follows from the stability of the Cu+1 rather than the other way around. Our results confirm the prediction that BaCu2As2 is an sp metal with weak electronic correlations. [ABSTRACT FROM AUTHOR]

Charge transport in nanocrystalline SiC with and without embedded Si nanocrystals (Si NCs) prepared by annealing of a-Si1-xCx:H precursors is studied using temperature-dependent current-voltage measurements supported by electron spin resonance and mass spectrometry data. Transport is Ohmic in all films at all temperatures and the temperature dependence of conductivity shows that the materials behave as disordered semiconductors, exhibiting extended-state transport at high temperature and variable-range hopping transport at low temperature. Grain-boundary-, surface-, and interface-dominated transport is systematically ruled out. Films are n type, and films with Si NCs exhibit up to 103 times higher conductivity (up to 0.1 S cm-1) after exposure to a hydrogen plasma which passivates dangling bonds, particularly on Si NCs. A forming gas anneal does not have such an effect, indicating that atomic rather than molecular hydrogen is required. The conductivity of SiC films without Si NCs is largely unchanged by passivation and the Fermi level is not raised nearly as closely to the conduction band. This is attributed to a type I band offset between Si NCs and SiC that leads to extended-state conduction in films with Si NCs taking place in a Si network. This is confirmed by the dependence of the extended-state mobility on the volume fraction of excess Si. Variable-range hopping is relatively insensitive to the presence of excess Si and is hence considered to take place via shallow defect states throughout the volume of the films. The high conductivities are found to be a consequence of background doping by oxygen and nitrogen. [ABSTRACT FROM AUTHOR]

A combination of ultraviolet and inverse photoemission is often used to determine the position of the transport levels of semiconductors. Although data from direct methods like photoemission appear advantageous at first glance, large discrepancies between thus-derived band gaps and optically measured band gaps have led to fundamentally different evaluation methods of the data from ultraviolet photoelectron spectroscopy (UPS)/inverse photoelectron spectroscopy (IPS) experiments, the essential alternatives being the maxima or the onsets of the frontier peaks. In this paper, we review published data as well as present new experimental data for a few representative II-VI and III-V compound and element semiconductors. New data from silicon are utilized as examples for evaluating details of such combined UPS and IPS spectra and for answering the question of how surface effects, especially the consequences of surface reconstruction, can adequately be taken into account. The results clearly indicate that, for all three types of semiconductors, only peak onsets represent the correct band positions. Possible reasons for this finding are discussed, and an explanation in the framework of relaxation (i.e., dynamical screening) is suggested. [ABSTRACT FROM AUTHOR]

Zigzag graphene nanoribbons have spin-polarized edges, antiferromagnetically coupled in the ground state with total spin zero. Customarily, these ribbons are made ferromagnetic by producing an imbalance between the two sublattices. Here we show that zigzag ribbons can be ferromagnetic due to the presence of reconstructed divacancies near one edge. This effect takes place even though the divacancies are produced by removing two atoms from opposite sublattices, which were balanced before reconstruction to 5-8-5 defects. We demonstrate that there is a strong interaction between the defect-localized and edge bands which mix and split away from the Fermi level. This splitting is asymmetric, yielding a net edge spin polarization. Therefore, the formation of reconstructed divacancies close to the edges of the nanoribbons can be a practical way to make them partially ferromagnetic. [ABSTRACT FROM AUTHOR]

We propose a new type of topological surface modes having cubic dispersion in three-dimensional topological superconductors. Lower order dispersions are prohibited by the threefold rotational symmetry and time-reversal symmetry. Cooper pairing in the bulk changes sign under improper rotations, akin to 3He-B. The surface manifestations are a divergent surface density of states at the Fermi level and isospins that rotate three times as they circle the origin in momentum space. We propose that Heusler alloys with band inversion are candidate materials to harbor the novel topological superconductivity. [ABSTRACT FROM AUTHOR]

We report hard x-ray photoemission spectroscopy measurements of the electronic structure of the prototypical correlated oxide SrxCa1-xVO3. By comparing spectra recorded at different excitation energies, we show that 2.2 keV photoelectrons contain a substantial surface component, whereas 4.2 keV photoelectrons originate essentially from the bulk of the sample. Bulk-sensitive measurements of the O 2p valence band are found to be in good agreement with ab initio calculations of the electronic structure, with some modest adjustments to the orbital-dependent photoionization cross sections. The evolution of the O 2p electronic structure as a function of the Sr content is dominated by A-site hybridization. Near the Fermi level, the correlated V 3d Hubbard bands are found to evolve in both binding energy and spectral weight as a function of distance from the vacuum interface, revealing higher correlation at the surface than in the bulk. [ABSTRACT FROM AUTHOR]

Making a metal contact to the two-dimensional semiconductor MoS2 without creating a Schottky barrier is a challenge. Using density functional calculations we show that, although the Schottky barrier for electrons obeys the Schottky-Mott rule for high work function (≳ 4.7 eV) metals, the Fermi level is pinned at 0.1-0.3 eV below the conduction band edge of MoS2 for low work function metals, due to the metal-MoS2 interaction. Inserting a boron nitride (BN) monolayer between the metal and the MoS2 disrupts this interaction, and restores the MoS2 electronic structure. Moreover, a BN layer decreases the metal work function of Co and Ni by ~2 eV, and enables a lineup of the Fermi level with the MoS2 conduction band. Surface modification by adsorbing a single BN layer is a practical method to attain vanishing Schottky barrier heights. [ABSTRACT FROM AUTHOR]

We report on theoretical investigations of the spin-orbit coupling effects in fluorinated graphene. First-principles density functional calculations are performed for the dense and dilute adatom coverage limits. The dense limit is represented by the single-side semifluorinated graphene, which is a metal with spin-orbit splittings of about 10 meV. To simulate the effects of a single adatom, we also calculate the electronic structure of a 10 × 10 supercell, with one fluorine atom in the top position. Since this dilute limit is useful to study spin transport and spin relaxation, we also introduce a realistic effective hopping Hamiltonian, based on symmetry considerations, which describes the supercell bands around the Fermi level. We provide the Hamiltonian parameters which are best fits to the first-principles data. We demonstrate that, unlike for the case of hydrogen adatoms, fluorine's own spin-orbit coupling is the principal cause of the giant induced local spin-orbit coupling in graphene. The sp³ hybridization induced transfer of spin-orbit coupling from graphene's of bonds, important for hydrogenated graphene, contributes much less. Furthermore, the magnitude of the induced spin-orbit coupling due to fluorine adatoms is about 1000 times more than that of pristine graphene, and 10 times more than that of hydrogenated graphene. Also unlike hydrogen, the fluorine adatom is not a narrow resonant scatterer at the Dirac point. The resonant peak in the density of states of fluorinated graphene in the dilute limit lies 260 meV below the Dirac point. The peak is rather broad, about 300 meV, making the fluorine adatom only a weakly resonant scatterer. [ABSTRACT FROM AUTHOR]

Angle-resolved photoemission spectroscopy (ARPES) is typically used to study only the occupied electronic band structure of a material. Here we use laser-based ARPES to observe a feature in bismuth-based superconductors that, in contrast, is related to the unoccupied states. Specifically, we observe a dispersive suppression of intensity cutting across the valence band, which, when compared with relativistic one-step calculations, can be traced to twofinal-state gaps in the bands 6 eV above the Fermi level. This finding opens up possibilities to bring the ultrahigh momentum resolution of existing laser-ARPES instruments to the unoccupied electron states. For cases where the final-state gap is not the object of study, we find that its effects can be made to vanish under certain experimental conditions. [ABSTRACT FROM AUTHOR]

The cerium α-γ phase transition is characterized by means of a many-body Jastrow-correlated wave function, which minimizes the variational energy of the first-principles scalar-relativistic Hamiltonian, and includes correlation effects in a nonperturbative way. Our variational ansatz accurately reproduces the structural properties of the two phases, and proves that even at temperature T = 0 K the system undergoes a first-order transition, with ab initio parameters which are seamlessly connected to the ones measured by experiment at finite T. We show that the transition is related to a complex rearrangement of the electronic structure, with a key role played by the p - f hybridization. The underlying mechanism unveiled by this work can hold in many Ce-bearing compounds, and more generally in other f-electron systems. [ABSTRACT FROM AUTHOR]

Electron interaction in nanoscale metal structures is of major interest in understanding and designing nanoscale electronic devices. In this study, electron interaction in epitaxially grown 20-nm-thick copper film on Si(100) was studied using femtosecond pump-probe technique with tunable probe photon energy near the d-band to Fermi-level transition. It was found that probe-photon-energy dependence on the transient reflectivity signal can be divided into four regimes, and the borders of these regimes depend on the pump power. Based on a phenomenological model, the excited electrons were separated into nonthermalized and thermalized electrons, and it was seen that the probe-photon-energy dependence of the signal can be explained by the difference of the ratios of nonthermalized to thermalized electron components. Furthermore, it was found that each of these two components can be selectively excited by properly choosing the probe photon energy. Each electron-phonon (el-ph) coupling and electron internal thermalization time were investigated individually without interference from each other's processes. We show that el-ph coupling factor is larger in epitaxial copper film than the previously reported value in polycrystalline-copper film. In addition, the electron internal thermalization time is smaller than the reported value for the polycrystalline gold and silver films. [ABSTRACT FROM AUTHOR]

We report a systematic polarization-dependent angle-resolved photoemission spectroscopy study of the threedimensional electronic structure of the recently discovered 112-type iron-based superconductor Ca1-xLaxFeAs2 (x = 0 .1 ). Besides the commonly reported three holelike and two electronlike bands in iron-based superconductors, we resolve one additional holelike band around the zone center and one more fast-dispersing band near the X point in the vicinity of the Fermi level. By tuning the polarization and the energy of incident photons, we are able to identify the specific orbital character and the kz dependence of all bands. Combining these results with band calculations, we find that As 4pz and 4px (4py) orbitals contribute significantly to the additional three-dimensional holelike band and the narrow band, respectively. Also, there is considerable hybridization between the As 4pz and Fe 3d orbitals in the additional holelike band, which suggests strong coupling between the unique arsenic zigzag bond layers and the FeAs layers therein. Our findings provide a comprehensive picture of the orbital character of the low-lying band structure of 112-type iron-based superconductors, which can be a starting point for the further understanding of their unconventional superconductivity. [ABSTRACT FROM AUTHOR]

A two-dimensional compound made of one monolayer of T1 and one monolayer of Sn on Si( 111) has been found to have a sandwichlike structure in which the Sn layer (having the milk-stool arrangement) resides on the bulklike terminated Si(l 11) surface and the T1 layer (having the honeycomb-chained-trimer arrangement) is located above the Sn layer. The electronic band structure of the compound contains two spin-split surface-state bands, of which one is nonmetallic and the other is metallic. Near the Fermi level the metallic band is split with the momentum splitting Δk11 = 0.037 Å-1 and energy splitting ΔEF = 167 meV. The steep dispersion of the band when crossing the Fermi level corresponds to an electron velocity of ≈8.5 × 105 m/s, which is comparable to the value reported for graphene. The 2D Fermi contours have almost circular shape with spin texture typical for hexagonal surfaces. [ABSTRACT FROM AUTHOR]

We study strongly correlated helical liquids with and without Rashba coupling using quantum Monte Carlo simulations of the Kane-Mele model with a Hubbard interaction at the edge. Independent of the Rashba coupling, we find that interactions enhance spin correlations and suppress the spectral weight at the Fermi level. For sufficiently strong interactions, a gap can be observed in the single-particle spectral function. However, based on a finite-size scaling analysis and theoretical arguments, we argue that this gap is closed by order parameter fluctuations in the Luttinger liquid phase even at zero temperature, and filled in by thermally induced kinks in the order parameter in the Mott phase at finite temperatures. While the bosonization suggests an umklapp-driven Mott transition only in the presence of Rashba coupling and hence a significant impact of the latter, our numerical results are almost unaffected by Rashba coupling even at low temperatures. [ABSTRACT FROM AUTHOR]

We present resonant Raman scattering experiments on nanotube samples enriched in metallic armchair single-wall carbon nanotubes (SWCNTs). We establish the transverse optical (ATO) phonon frequency for the (5,5) through (10,10) armchair species, ranging in diameter from 0.68 to 1.36 nm. The frequencies show a strong diameter dependence similar to that previously observed in semiconducting nanotubes. We show that the ATO frequencies in armchair SWCNTs are dramatically upshifted from those of semiconducting SWCNTs. Furthermore, using electrochemical doping, we demonstrated that the ATO frequencies in armchair SWCNTs are independent of the position of the Fermi level. These results suggest that the upshift is a result of a Kohn anomaly involving a forward-scattering mechanism of electrons close to the Fermi level. This is in contrast to the well-known Kohn anomaly that dominates the downshift of the ALO and E2g phonons in nonarmchair metallic SWCNTs and graphene, respectively. [ABSTRACT FROM AUTHOR]

The Fermi level of a material is a fundamental quantity that determines its electronic properties. Thus, the ability to tune Fermi levels is important for developing electronic device materials. However, for most materials, the Fermi level is limited to a certain range in the band gap due to the existence of certain intrinsic compensating defects. Here we demonstrate that quenching can be used as an effective way to overcome this limit, allowing the Fermi levels to be tuned in a much wider range. Taking a photovoltaic material CdTe as a prototype example, we analyzed the physical origin of Fermi level pinning and explained why growing the sample at high temperature followed by rapid quenching to room temperature can overcome the self-compensation limit. We further show that for CdTe, quenching can increase the Fermi level range from about 0.6 to 1.1 eV, which has a great potential in improving CdTe solar cell performance. Our proposed strategy of tuning Fermi level positions beyond the intrinsic equilibrium doping limit is general and can be applied to other semiconductor systems. [ABSTRACT FROM AUTHOR]

We study the effects of strong coupling of a localized state charge to one-dimensional electronic channels out of equilibrium. While the state of this charge and the coupling strengths determine the scattering phase shifts in the channels, the nonequilibrium partitioning noise induces the tunneling transitions to the localized state. The strong coupling leads to a nonperturbative backaction effect which is manifested in the orthogonality catastrophe and the Fermi-edge singularity in the transition rates. We predict an unusually pronounced manifestation of the non-Gaussian component of noise that breaks the charge symmetry, resulting in a nontrivial shape, and a shift of the position of the tunneling resonance. [ABSTRACT FROM AUTHOR]

We study the Hubbard model on the honeycomb lattice with nearest-neighbor hopping (t>0) and next-nearest-neighbor hopping (t'<0). When t'<−t/6, the single-particle spectrum is featured by the continuously distributed Van Hove saddle points at the band bottom, where the density of states diverges in a power law. We investigate possible unconventional superconductivity in such systems with the Fermi level close to the band bottom by employing both random-phase-approximation and determinant quantum Monte Carlo approaches. Our study reveals a possible triplet p+ip superconductivity in this system with appropriate interactions. Our results might provide a possible route to look for triplet superconductivity with relatively high transition temperature in low-filled graphene and other similar systems. [ABSTRACT FROM AUTHOR]

The electronic and spin structure of a graphene monolayer synthesized on Pt(111) has been investigated experimentally by angle- and spin-resolved photoemission with different polarizations of incident synchrotron radiation and using density functional theory calculations. It is shown that despite the observed total quasifreestanding character of the dispersion of the graphene π state remarkable local distortions and breaks in the dispersions take place due to hybridization between the graphene π and Pt d states. Corresponding spin-dependent avoided-crossing effects lead to significant modification of the spin structure and cause an enhanced induced spin-orbit splitting of the graphene π states near the Fermi level in the region of the K¯¯¯ point of the graphene Brillouin zone (BZ) with a magnitude of 80–200 meV depending on the direction in the BZ. Using p, s, and elliptical polarizations of the synchrotron radiation, the contributions of the graphene π and Pt d states were separated and their intersection at the Fermi level, which is important for effective spin injection between these states, was shown. Moreover, analysis of the data allows us to conclude that in the region of the Dirac point the spin structure of the system cannot be described by a Rashba splitting, and even a spin-orbit gap between lower and upper Dirac cones is observed. [ABSTRACT FROM AUTHOR]

We theoretically analyse the process of charge recombination in the planar Mott-Hubbard insulators with the aim to explain short picosecond-range lifetime of photoexcited carriers, experimentally studied via pump-probe experiments on the undoped cuprates. The recombination mechanism consists of two essential ingredients: the formation of a metastable s-type bound holon-doublon pair, i.e. the Mott exciton, and the decay of such an excitonic state via the multimagnon emission. In spite of the large gap that requires many bosons to be emitted, latter process is fast due to large exchange scale and strong charge-spin coupling in planar systems. As the starting microscopic model we consider the single-band Hubbard model, and then more realistic three-band model for cuprates, both leading to the same minimal one. The decay rate of the exciton is evaluated numerically via the Fermi golden rule, having consistency also with the direct time-evolution calculation. The decay rate reveals exponential dependence on the ratio of the Mott-Hubbard gap and the exchange coupling - the result qualitatively reproduced also within a toy exciton-boson model. [ABSTRACT FROM AUTHOR]

We report ballistic transport commensurability minima in the magnetoresistance of ν=3/2 composite fermions (CFs). The CFs are formed in high-quality two-dimensional electron systems confined to wide GaAs quantum wells and subjected to an in-plane, unidirectional periodic potential modulation. We observe a slight asymmetry of the CF commensurability positions with respect to ν=3/2, which we explain quantitatively by comparing three CF density models and concluding that the ν=3/2 CFs are likely formed by the minority carriers in the upper energy spin state of the lowest Landau level. Our data also allow us to probe the shape and size of the CF Fermi contour. At a fixed electron density of ≃1.8×1011 cm−2, as the quantum well width increases from 30 to 60 nm, the CFs show increasing spin polarization. We attribute this to the enhancement of the Zeeman energy relative to the Coulomb energy in wider wells where the latter is softened because of the larger electron layer thickness. The application of an additional parallel magnetic field (B∥) leads to a significant distortion of the CF Fermi contour as B∥ couples to the CFs' out-of-plane orbital motion. The distortion is much more severe compared to the ν=1/2 CF case at comparable B∥. Moreover, the applied B∥ further spin-polarizes the ν=3/2 CFs as deduced from the positions of the commensurability minima. [ABSTRACT FROM AUTHOR]

Much interest in the superconducting proximity effect in three-dimensional (3D) topological insulators (TIs) has been driven by the potential to induce Majorana bound states at the interface. Most candidate materials for 3D TI, however, are bulk metals, with bulk states at the Fermi level coexisting with well-defined surface states exhibiting spin-momentum locking. In such topological metals, the proximity effect can differ qualitatively from that in TIs. By studying a model topological metal-superconductor (TM-SC) heterostructure within the Bogoliubov-de Gennes formalism, we show that the pair amplitude reaches the naked surface, unlike in a topological insulator-superconductor (TI-SC) heterostructure where it is confined to the interface. Furthermore, we predict vortex-bound-state spectra to contain a Majorana zero mode localized at the naked surface, separated from the bulk vortex-bound-state spectra by a finite energy gap in such a TM-SC heterostructure. These nakedsurface- bound modes are amenable to experimental observation and manipulation, presenting advantages of TM-SC over TI-SC. [ABSTRACT FROM AUTHOR]

We present a comprehensive ab initio study of structural, electronic, lattice dynamical, and electron-phonon coupling properties of the Bi(111) surface within density functional perturbation theory. Relativistic corrections due to spin-orbit coupling are consistently taken into account. Changes of interatomic couplings are confined mostly to the first two bilayers, resulting in superbulk modes with frequencies higher than the optic bulk spectrum, and in an enhanced density of states at lower frequencies for atoms in the first bilayer. We give results for the momentum-dependent electron-phonon coupling of electronic states belonging to the two surface electronic bands along ΓM which cross the Fermi energy. For larger momenta, the lower surface band exhibits a moderate electron-phonon coupling of about 0.45, which is larger than the coupling constant of bulk Bi. For momenta close to Γ, states of both surface bands show even stronger couplings because of interband transitions to bulk states near Γ around the Fermi level. For these cases, the state-dependent Eliashberg functions exhibit pronounced peaks at low energy and strongly deviate in shape from a Debye-type spectrum, indicating that an extraction of the coupling strength from measured electronic self-energies based on this simple model is likely to fail. [ABSTRACT FROM AUTHOR]

An exact mapping of the tight-binding Hamiltonian for a graphene nanoribbon under any armchair uniaxial strain into an effective one-dimensional system is presented. As an application, for a periodic modulation we have found a gap opening at the Fermi level and a complex fractal spectrum, akin to the Hofstadter butterfly resulting from the Harper model. The latter can be explained by the commensurability or incommensurability nature of the resulting effective potential. When compared with the zig-zag uniaxial periodic strain, the spectrum shows much bigger gaps, although in general the states have a more extended nature. For a special critical value of the strain amplitude and wavelength, a gap is open. At this critical point, the electrons behave as relativistic Dirac fermions in one direction, while, in the other direction, a nonrelativistic Schrödinger behavior is observed. Also, some topological states were observed which have the particularity of not being completely edge states since they present some amplitude in the bulk. However, these are edge states of the effective system due to a reduced dimensionality through decoupling. These states also present the fractal Chern beating observed recently in quasiperiodic systems. [ABSTRACT FROM AUTHOR]

The Rashba effect is interesting for thermoelectrics because of the unique spin-splitting band structure. By using bulk BiTel with a giant Rashba effect as an example, we prove that the spin-splitting-induced constant density of states leads to a two-dimensional thermopower. The thermopower is higher as compared with that in spin-degenerate bands, primarily due to the lower Fermi level in the Rashba spin-splitting bands at given carrier concentrations. A quantitative relation between thermopower and the Rashba parameter is established. Furthermore, the internal electric field in the Rashba system can be beneficial to bond anharmonicity and low lattice thermal conductivity. We suggest that bulk materials with large Rashba effect may become potential candidates for efficient thermoelectricity. [ABSTRACT FROM AUTHOR]

Quantum oscillation measurements provide relevant information about the Fermi surface (FS) properties of strongly correlated metals. Here, we report on the Shubnikov-de Haas effect via high-field resistivity measurements of EuFe2As2 (Eul22) and BaFe2As2 (Bal22) single crystals. Although both pnictide compounds are isovalent with similar effective masses and density of states, at the Fermi level, our results reveal subtle changes in their fermiology. Remarkably, although the spin-density-wave (SDW) ordering temperature is higher in the Eu-rich end, Eul22 displays a much more isotropic and three-dimensional-like FS when compared with Bal22, in agreement with band structure calculations. Our experimental results suggest an anisotropic contribution of the Fe 3d orbitals to the FS in Bal22. We speculate that this orbital differentiation may be responsible for the suppression of the SDW phase in the FeAs-based compounds. [ABSTRACT FROM AUTHOR]

The electronic structure of phase separated KxFe2-ySe2 has been studied by means of x-ray absorption spectroscopy (XAS) and photoemission spectroscopy. Both Fe L2,3-edge XAS and Fe 2p photoemission spectra show a signature of the metallic phase that is embedded in the insulating texture. Valence-band spectra, measured with different excitation energies, reveal a finite density of states at the Fermi level driven by the Fe 3d orbitals. A combined analysis of the Se L2,3-edge and the Se k-edge XAS provides fürther information on a significant density of states of Se 4p character near the Fermi level, consistent with a large hybridization of these states with the Fe 3d orbitals. The K L2 3-edge XAS spectrum is also presented to show element specific partial density of states. The results are discussed in connection with the electronic phase separation in KxFexFe2-ySe2 driven by the Fe 3d states. [ABSTRACT FROM AUTHOR]

We theoretically investigate the thermoelectric properties of a spin-polarized two-dimensional electron gas hosting a Kondo adatom hybridized with a STM tip. Such a setup is treated within the single-impurity Anderson model in combination with the atomic approach for the Green's functions. Due to the spin dependence of the Fermi wave numbers, the electrical and thermal conductances together with thermopower and Lorenz number reveal beating patterns as a function of the STM tip position in the Kondo regime. In particular, by tuning the lateral displacement of the tip with respect to the adatom vicinity, the temperature, and the position of the adatom level, one can change the sign of the Seebeck coefficient through charge and spin. This opens a possibility of the microscopic control of the heat flux analogously to that established for the electrical current. [ABSTRACT FROM AUTHOR]