meV Gap Research Articles

Magnetic gap opening in rhombohedral-stacked multilayer graphene from first principles

We investigate the occurrence of magnetic and charge density wave instabilities in rhombohedral-stacked multilayer (three to eight layers) graphene by first principles calculations including exact exchange. Neglecting spin-polarization, an extremely flat surface band centered at the special point ${\bf K}$ of the Brillouin zone occurs at the Fermi level. Spin polarization opens a gap in the surface state by stabilizing an antiferromagnetic state. The top and the bottom surface layers are weakly ferrimagnetic in-plane (net magnetization smaller than $10^{-3}\mu_B$), and are antiferromagnetic coupled to each other. This coupling is propagated by the out-of-plane antiferromagnetic coupling between the nearest neighbors. The gap is very small in a spin-polarized generalized gradient approximation, while it is proportional to the amount of exact exchange in hybrid functionals. For trilayer rhombohedral graphene it is $38.6$ meV in PBE0, in agreement with the $42$ meV gap found in experiments. We study the temperature and doping dependence of the magnetic gap. At electron doping of $n \sim 7 \times 10^{11}$ cm$^{-2}$ the gap closes. Charge density wave instabilities with $\sqrt{3}\times\sqrt{3}$ periodicity do not occur.

Superconducting gap structure in ambient-pressure-grown LaO0.5F0.5BiS2

We have performed transverse-field muon spin relaxation (TF-$\mu$SR) measurements on ambient-pressure-grown polycrystalline $\mathrm{LaO_{0.5}F_{0.5}BiS_{2}}$. From these measurements, no signature of magnetic order is found down to 25 mK. The value of the magnetic penetration depth extrapolated to 0 K is 0.89 (5) $\mu$m. The temperature dependence of superconducting penetration depth is best described by either a multigap s + s-wave model with $\Delta_{1}$ = 0.947 (7) meV and $\Delta_{2}$ = 0.22 (4) meV or the ansiotropic s-wave model with $\Delta(0)$ = 0.776 meV and anisotropic gap amplitude ratio $\Delta_{min}/\Delta_{max}$ = 0.34. Comparisons with other potentially multigap $\mathrm{BiS_{2}}$-based superconductors are discussed. We find that these $\mathrm{BiS_{2}}$-based superconductors, including $\mathrm{Bi_{4}O_{4}S_3}$ and the high-pressure synthesized $\mathrm{LaO_{0.5}F_{0.5}BiS_{2}}$, generally conform to the Uemura relation.

Spin wave excitations in the tetragonal double perovskiteSr2CuWO6

Sr$_2$CuWO$_6$ is a double perovskite proposed to be at the border between two and three dimensional magnetism, with a square lattice of $S=\frac{1}{2}$ Cu$^{2+}$ ions. We have used inelastic neutron scattering to investigate the spin wave excitations of the system, to find out how they evolve as a function of temperature, as well as to obtain information about the magnetic exchange interactions. We observed well defined dispersive spin wave modes at $6$~K, which partially survive above the magnetic ordering temperature, $T_N=24$~K. Linear spin wave theory is used to determine the exchange interactions revealing them to be highly two-dimensional in nature. Density functional theory calculations are presented supporting this experimental finding, which is in contrast to a previous \emph{ab-initio} study of the magnetic interactions. Our analysis confirms that not the nearest neighbour, but the next nearest neighbour interactions in the tetragonal $ab$ plane are the strongest. Low incident energy measurements reveal the opening of a $0.6(1)$~meV gap below $T_N$, which suggests the presence of a very weak single ion anisotropy term in the form of an easy axis along $\hat{\mathbf{a}}$.

Influence of Spin-Orbit Coupling on Electronic Structure of Polyyne and Cumulene Carbynes

ABSTRACTThe present paper has suggested a new non-observational method to calculate electronic structure of carbynes taking into consideration the influence of the spin-orbital coupling. The method is demonstrated by calculations of the structure splitting at the Fermi level in cumulene and polyyne carbynes having semiconductor and metallic electronic structure correspondingly. These couplings result in 2 - 3 meV gaps.

Field-induced quantum metal–insulator transition in the pyrochlore iridate Nd2Ir2O7

A combination of strong spin–orbit coupling and electronic correlations in pyrochlore iridates produces a quantum insulator–metal transition that can be induced by applying a magnetic field along specific crystalline axes. The metal–insulator transition (MIT) is a hallmark of strong correlation in solids1,2,3. Quantum MITs at zero temperature have been observed in various systems tuned by either carrier doping or bandwidth1. However, such transitions have rarely been induced by application of magnetic field, as normally the field scale is too small in comparison with the charge gap, whose size is a fraction of the Coulomb repulsion energy (∼1 eV). Here we report the discovery of a quantum MIT tuned by a field of ∼10 T, whose magnetoresistance exceeds 60,000%. In particular, our anisotropic magnetotransport measurements on the cubic insulator Nd2Ir2O7 (ref. 4) reveal that the insulating state can be suppressed by such a field to a zero-temperature quantum MIT, but only for fields near the [001] axis. The strong sensitivity to the field direction is remarkable for a cubic crystal, as is the fact that the MIT can be driven by such a small magnetic field, given the 45 meV gap energy5, which is of order of 50 times the Zeeman energy for an Ir4+ spin. The systematic change in the MIT from continuous near zero field to first order under fields indicates the existence of a tricritical point proximate to the quantum MIT. We argue that these phenomena imply both strong correlation effects on the Ir electrons and an active role for the Nd spins.

Emergent surface superconductivity in the topological insulator Sb2Te3

Surfaces of three-dimensional topological insulators have emerged as one of the most remarkable states of condensed quantum matter where exotic electronic phases of Dirac particles should arise. Here we report on superconductivity in the topological insulator Sb2Te3 with transition to zero resistance induced through a minor tuning of growth chemistry that depletes bulk conduction channels. The depletion shifts Fermi energy towards the Dirac point as witnessed by a factor of 300 reduction of bulk carrier density and by the largest carrier mobility (≳25,000 cm(2) V(-1) s(-1)) found in any topological material. Direct evidence from transport, the unprecedentedly large diamagnetic screening, and the presence of ∼25 meV gaps detected by scanning tunnelling spectroscopy reveal the superconducting condensate to emerge first in surface puddles, with the onset of global phase coherence at ∼9 K. The rich structure of this state lends itself to manipulation via growth conditions and the material parameters such as Fermi velocity and mean free path.

Indirect detection of dark matter using MeV-range gamma-ray telescopes

The astrophysics community is considering plans for a variety of gamma-ray telescopes (including ACT and GRIPS) in the energy range 1--100 MeV, which can fill in the so-called "MeV gap" in current sensitivity. We investigate the utility of such detectors for the study of low-mass dark matter annihilation or decay. For annihilating (decaying) dark matter with a mass below about 140 MeV (280 MeV) and couplings to first generation quarks, the final states will be dominated by photons or neutral pions, producing striking signals in gamma-ray telescopes. We determine the sensitivity of future detectors to the kinematically allowed final states. In particular, we find that planned detectors can improve on current sensitivity to this class of models by up to a few orders of magnitude.

Angle-resolved photoemission observation of isotropic superconducting gaps in isovalent Ru-substituted Ba(Fe0.75Ru0.25)2As2

We used high-energy resolution angle-resolved photoemission spectroscopy to extract the momentum dependence of the superconducting gap of Ru-substituted Ba(Fe$_{0.75}$Ru$_{0.25}$)$_2$As$_2$ ($T_c = 15$ K). Despite a strong out-of-plane warping of the Fermi surface, the magnitude of the superconducting gap observed experimentally is nearly isotropic and independent of the out-of-plane momentum. More precisely, we respectively observed 5.7 meV and 4.5 meV superconducting gaps on the inner and outer $\Gamma$-centered hole Fermi surface pockets, whereas a 4.8 meV gap is recorded on the M-centered electron Fermi surface pockets. Our results are consistent with the $J_1-J_2$ model with a dominant antiferromagnetic exchange interaction between the next-nearest Fe neighbors.

Tunneling investigation of the electron scattering effect on the momentum-dependent energy gap distribution in MgB2

We have studied the effect of electron scattering on the momentum-dependent energy gap distributions in MgB2 by measuring planar tunnel junctions made from epitaxial MgB2 thin films on different substrates, of different thicknesses, with different junction interface properties, and different counter-electrode materials. The phonon features in the tunneling spectra indicated that the native oxide barrier is mainly MgO with estimated barrier thickness ranging from 1.1 to 1.5 nm and the average barrier height from 1.7 to 2.6 eV. When tunneling into the ab-plane occurred in very clean films, both the π (∼1.8 meV) and σ (∼7.2 meV) gaps were observed with fine structures in the conductance peaks, indicating a distribution of gap values due to anisotropic electron-phonon interaction. The σ gap was enhanced (∼7.9 meV) in MgB2 thin films on SiC substrates which had Tc values over 40 K due to epitaxial tensile strain. As the MgB2 film thickness decreased from 100 nm to 33 nm, the π gap distribution range was narrowed from ∼1.7 meV to ∼1.4 meV, and the fine peak structures disappeared due to the reduced electron mean free path limited by the film thickness. The fine peak structures were also gradually smeared out when the junction resistance increased. The results show that the electron scattering, either from the MgB2 film or the junction interface or barrier, can smear out the gap distribution structures in the tunneling spectra. Deterioration of the MgB2 film surface was also found to cause an increase in the π gap value, likely due to an enhancement of interband scattering.

Hybrid Graphene and Graphitic Carbon Nitride Nanocomposite: Gap Opening, Electron–Hole Puddle, Interfacial Charge Transfer, and Enhanced Visible Light Response

Opening up a band gap and finding a suitable substrate material are two big challenges for building graphene-based nanodevices. Using state-of-the-art hybrid density functional theory incorporating long-range dispersion corrections, we investigate the interface between optically active graphitic carbon nitride (g-C(3)N(4)) and electronically active graphene. We find an inhomogeneous planar substrate (g-C(3)N(4)) promotes electron-rich and hole-rich regions, i.e., forming a well-defined electron-hole puddle, on the supported graphene layer. The composite displays significant charge transfer from graphene to the g-C(3)N(4) substrate, which alters the electronic properties of both components. In particular, the strong electronic coupling at the graphene/g-C(3)N(4) interface opens a 70 meV gap in g-C(3)N(4)-supported graphene, a feature that can potentially allow overcoming the graphene's band gap hurdle in constructing field effect transistors. Additionally, the 2-D planar structure of g-C(3)N(4) is free of dangling bonds, providing an ideal substrate for graphene to sit on. Furthermore, when compared to a pure g-C(3)N(4) monolayer, the hybrid graphene/g-C(3)N(4) complex displays an enhanced optical absorption in the visible region, a promising feature for novel photovoltaic and photocatalytic applications.

Electronic structure of graphene on single-crystal copper substrates

The electronic structure of graphene on Cu(111) and Cu(100) single crystals is investigated using low-energy electron microscopy, low-energy electron diffraction, and angle-resolved photoemission spectroscopy. On both substrates the graphene is rotationally disordered and interactions between the graphene and substrate lead to a shift in the Dirac crossing of $\ensuremath{\sim}$$\ensuremath{-}$0.3 eV and the opening of a $\ensuremath{\sim}$250 meV gap. Exposure of the samples to air resulted in intercalation of oxygen under the graphene on Cu(100), which formed a ($\sqrt{2}\ifmmode\times\else\texttimes\fi{}2\sqrt{2}$)R45${}^{\mathrm{o}}$ superstructure. The effect of this intercalation on the graphene $\ensuremath{\pi}$ bands is to increase the offset of the Dirac crossing ($\ensuremath{\sim}$$\ensuremath{-}$0.6 eV) and enlarge the gap ($\ensuremath{\sim}$350 meV). No such effect is observed for the graphene on the Cu(111) sample, with the surface state at $\ensuremath{\Gamma}$ not showing the gap associated with a surface superstructure. The graphene film is found to protect the surface state from air exposure, with no change in the effective mass observed, as for one monolayer of Ag on Cu(111).

The effect of the orientation of the Stone–Wales defects on the bands structure of carbon nanotubes

We have studied the effect of orientation of Stone–Wales (SW) defects formed on the armchair single-wall carbon nanotubes (SWCNTs) on their metal-semiconductor behaviour by means of the density functional formalism. It has been shown that two third of possible orientations of SW defects, at high concentrations, open a ≈ 40 meV gap at Fermi level while in the other one third orientations tube remains metallic.

Influence of atomic hydrogen annealing on the optical properties of vacuum-deposited a-Si:H films

The influence of atomic hydrogen annealing on the optical parameters of a-Si:H films was studied using spectrophotometric measurements of the film transmittance and reflectance in the wavelength range 200–3000 nm. In this annealing, the deposition of a thin layer and treatment with atomic hydrogen were repeated alternately, where the thickness of the thin cyclic layer, d cyc, and the treatment time of each cycle, t ca, were kept fixed for each sample. A series of different samples with average thickness of 0.5 μm and different d cyc and t ca were prepared. It was found that the refractive index, n, and the optical energy gap, E g, increase as the treatment time, t ca, increases from 0 to 60 s, while at t ca=120 s both n and E g decrease. Also, both the refractive index and the optical energy gap decrease with increasing the relative diffusion length of hydrogen, √ t ca/ d cyc from 0.39 to 0.77. The widening of E g is due to the structural relaxation resulting from impingement of atomic hydrogen on the growing surface. Thus, a good-quality a-Si:H with Urbach parameter 65 mev and optical energy gap of 1.78 eV was successfully prepared.

Studying Single-Wall Carbon Nanotubes Through Encapsulation: From Optical Methods Till Magnetic Resonance

Encapsulating fullerenes, magnetic fullerenes, 13C isotope enriched fullerenes, and organic solvents inside SWCNTs enables to yield unprecedented insight into their electronic, optical, and interfacial properties and to study SWCNT growth. In addition to customary methods of their studies such as e.g., optical absorption or Raman spectroscopy, these efforts enables to employ electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. Encapsulated C60 fullerenes are transformed to inner tubes by a high temperature annealing. The diameter distribution of the inner tubes follow that of the outer ones and their unique, low defect concentration makes them an ideal model system for high resolution and energy dependent Raman studies. The observation of Raman modes of individual inner-outer tube pairs allows to measure the inner-outer tube interaction strength that is also well described theoretically. Reversible closing and opening of SWCNT can be studied in a diameter selective manner by encapsulating C60 and transforming it to an inner tube. The growth of inner tubes can be achieved from 13C enriched encapsulated organic solvents, which shows that the geometry of the fullerene does not play a particular role in the inner tube growth as it was originally thought. In addition, it opens new perspectives to explore the in-the-tube chemistry. Growth of inner tubes from 13C enriched fullerenes provides a unique isotope engineered heteronuclear system, where the outer tubes contain natural carbon and the inner walls are controllably 13C isotope enriched. The material enables to identify the vibrational modes of inner tubes which otherwise strongly overlap with the outer tube modes. The 13C NMR signal of the material has an unprecedented specificity for the small diameter SWCNTs. Temperature and field dependent 13C T1 studies show a uniform metallic-like electronic state for all inner tubes rather than distributed metallic and isolating behavior. A low energy, 3 meV gap is observed that is tentatively assigned to a long sought Peierls transition in the small diameter SWCNTs. Encapsulating magnetic fullerenes, such as N@C60 and C59N opens the way for local probe ESR studies of the electronic properties of the SWCNTs.

Multiple-gap structure of the binary superconductor MgB2

Electron-tunneling measurements on the superconductor MgB2 with Tc=39 K have been carried out using a break-junction technique. We have observed the double-gap structures with several combinations of the gap sizes. The largest Δ=9 meV gap can be observed as a single gap, which gives very strong-coupling ratio 2Δ/kBTc=5.3–5.7.

Nanoscale One-Dimensional Scattering Resonances in the CuO Chains ofYBa2Cu3O6+x

We present scanning tunneling spectroscopy measurements of the CuO chain plane in YBa(2)Cu(3)O(6+x), showing an approximately 25 meV gap in the local density of states (LDOS) filled by numerous intragap resonances: intense peaks in LDOS spectra associated with one-dimensional, Friedel-like oscillations. We discuss how these phenomena shed light on recent results from other probes, as well as their implications for phenomena in the superconducting CuO(2) plane.

Scanning probe microscopy and spectroscopy study of the organic salt (ET)2KHg(SCN)4

The surfaces of cleaved single crystals of (ET)2KHg(SCN)4 [where ET is bis(ethylenedithio)tetrathiafulvalene] were imaged by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Two distinct surfaces are observed with STM: a complete (ET)+2 layer and a complete KHg(SCN)−4 layer. STM images of the cation layer are independent of scanning tip polarity, which is consistent with its metallic character. By contrast, images of the anion layer are strongly dependent upon the polarity of the tip and thus provide information about occupied and unoccupied molecular orbitals in the KHg(SCN)−4 surface. The AFM images were obtained at ambient conditions and correlate well with STM images of the (ET)+2 layer. Preliminary spectroscopy measurements on cleaved crystals at low temperature indicate that the insulating ground state phase contains a 3 meV gap in the density of states, which has a value of 2Δ/kTc=8.

Local order, epitaxy, and electronic structure of the Bi/III–V semiconductor interfaces

The growth morphology and electronic structure of the Bi/InP(110) interface is investigated and compared to the Bi/GaAs(110) system. High-resolution synchrotron radiation core level and valence band photoelectron spectroscopy is employed in the determination of the local order, epitaxy, and electronic structure development for these systems. Valence band spectra show the development of a density of states between the InP valence band maximum and the Fermi level in the coverage range 0–1 monolayer. These Bi-derived states show a 450 meV gap between the onset of emission and the Fermi level indicating the first monolayer is semiconducting. At all coverages beyond 1 monolayer, the Bi-derived states show emission at the Fermi level. Core-level intensity attenuation profiles indicate a Stranski–Krastanov growth mode. Computer fitting of the Bi 5d core level shows two bonding sites for Bi on the surface, with a third Bi 5d component at all coverages above 1 monolayer. A zigzag chain model used for the Bi/GaAs(110) system is proposed as an appropriate model for the Bi/InP(110) interface.

A spectroscopic study of some high‐temperature superconductors using a low‐temperature STM

SUMMARYWe used a scanning tunnelling microscope (STM) to measure both the tunnel current, I, and the dynamic conductance, dI/dV, at 4·2 K for a number of high‐transition temperature oxide superconductors. Large spatial variations in the tunnelling characteristics are observed. At low tunnel resistances, all samples show evidence of single electron tunnelling and incremental charging. Results on BiSrCaCu2Ox show the coexistence of charging with Josephson coupling between grains within the sample. Results on both the Bi sample and a single crystal of YBa2Cu3O6·5+x reveal possible energy gap (2A) values of 17 and 20 meV, respectively. A very sharp 5 meV gap, observed in ceramic samples of YBa2Cu3O6·5+x and Y0·5Al0·05Ba2Cu3O6·5+x, may indicate the presence of a lower temperature phase in these samples.

Galvanomagnetic properties of lead-telluride quantum wells

The first in-plane galvanomagnetic measurements on PbTe quantum wells are presented. The samples were 70-Å-wide wells sandwiched between lattice-matched PbEuSeTe layers which have 500 meV gaps. PbTe has a 187 meV gap at 0 K. The magnetoresistance and Hall effect were measured from 0 to 22 T and 0.5 to 300 K. The results can be explained if we assume that the wells are modulation doped p type to 3.7×1013 cm−2, and that there are 3×1012 cm−2 highly mobile holes which show Shubnikov–de Haas oscillations due to the splitting of size-quantized subbands into Landau levels. The influence of illumination is also shown: up to 1014 cm−2 carriers with a lifetime of the order of 102 min can be photoexcited in the structure.