The study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at ∆1=2.3 meV and ∆2=7.1 meV. Different spectral weights of the partial superconducting density of states (PDOS) are a reflection of different tunneling directions in this multi-band system. Our experimental observations are consistent with the existence of two-band superconductivity in the presence of interband superconducting pair interaction and quasiparticle scattering. Temperature evolution of the tunneling spectra follows the BCS scenario [1] with both gaps vanishing at the bulk Tc. Indeed, the study of tunneling junctions exhibiting only the small gap (c-axis tunneling) clearly and reproducibly show that this gap persists up to the bulk Tc. The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al. [2] .The discovery of superconductivity in MgB 2 [3] at 39K sparked great interest in the fundamental physics and practical applications of this material. There has already been rapid progress in understanding the physical properties of this superconductor. Specific heat measurements [4,5] show that MgB 2 is an s-wave superconductor and the presence of the isotope effect [6,7] points towards phonon-mediated pairing. Tunneling and photoemission spectroscopy directly measures the superconducting energy gap and can provide further understanding of the origin of the superconductivity in this material. Earlier tunneling spectroscopy measurements show a large spread in the gap values [8][9][10] each consistent with the BCS form. More recent experiments, including STM tunneling spectroscopy [11], point-contact spectroscopy [12,13], specific heat measurements [4,5], and Raman spectroscopy [14] point towards the existence of two distinct gaps. This scenario has been predicted theoretically by Liu et al. [2]. First principle calculations show that the Fermi surface of MgB 2 consists of 2D cylindrical sheets arising from σ antibonding states of B p xy orbitals, and 3D tubular networks arising from π bonding and antibonding states of B p z orbitals. In this theoretical framework [2] two different energy gaps exist, the smaller one being an induced gap associated with the 3D bands and the larger one associated with the superconducting 2D bands. Furthermore both superconducting gaps should vanish at the bulk critical temperature T c . Due to this highly anisotropic band structure the superconducting gaps should be momentum-dependent reflecting the strength of the electron-phonon coupling of the carriers in the different bands. Up to now there has been no direct experimental evidence of the orientation dependence of the order parameter in this material. Moreover, the temperature dependence of the two gaps would give further insights into the nature of superconductivity in MgB 2 . Scanning tunneling spectroscopy is a unique technique that allows direct measure of the DOS near the Fermi energy with high...

We present scanning tunneling microscopy measurements of the surface of superconducting MgB2 with a critical temperature of 39K. In zero magnetic field the conductance spectra can be analyzed in terms of the standard BCS theory with a smearing parameter Γ. The value of the superconducting gap is 5.2 meV at 4.2 K, with no experimentally significant variation across the surface of the sample. The temperature dependence of the gap follows the BCS form, fully consistent with phonon-mediated superconductivity in this novel superconductor. The application of a magnetic field induces strong pair-breaking as seen in the conductance spectra in fields up to 6 T.One route for finding new superconducting compounds with a high critical temperature combines light elements with high ionicity, a large density of electronic states at the Fermi level, and stiff elastic response due to high frequency phonon modes. Many carbides and nitrides fall into this category, and some of them show superconducting transition temperatures as high as 10-20 K. The recent discovery of superconductivity at 40 K in MgB 2 [1] reinvigorates strong interest in this approach.MgB 2 has a remarkably high T c for a simple binary compound. The chemical structure is quite simple as well, consisting of alternating hexagonal layers of Mg atoms and boron honeycomb layers. Band structure calculations [2,3] show that this compound is ionic, has a high density of states at the Fermi level, high phonon frequencies, and strong electron-phonon interactions. These features favor a high superconducting transition temperature arising from phonon mediated electron pairing. The presence of the isotope effect [4] confirms the important role of phonons in the superconductivity of this compound. Transport and magnetic measurements [5][6][7][8][9] are beginning to probe the macroscopic response of the superconducting and normal states, and the supercondicting performance for applications is being evaluated [11,12].Here we report the superconducting energy gap of polycrystalline MgB 2 pellets as seen in scanning tunneling spectroscopy. These measurements directly probe the quasiparticle excitations near the Fermi energy and provide key information on the nature of the superconducting energy gap and its temperature and field dependence. We achieve clean vacuum tunneling with conductance spectra that are identical within experimental error across the sample scan area. The tunneling spectroscopy at 4.2 K is consistent with the modified BCS density of states [13] (DOS) with a superconducting gap value of 5.2 meV and pair-breaking strength Γ of 3 meV. From STM measurements we show that the temperature dependence of the gap follows a BCS behavior. In addition, we present the magnetic field dependence of the tunneling spectra at 4.2 K showing the pair-breaking effect of the magnetic field in this material.The MgB 2 sample was synthesized from a high purity, 3 mm diameter Mg rod and isotopic 11 B (Eagle Picher, 98.46 atomic % 11 B). The Mg rod was cut into pieces about 4 mm long and ...

A charge-density wave (CDW) state has a broken symmetry described by a complex order parameter with an amplitude and a phase. The conventional view, based on clean, weak-coupling systems, is that a finite amplitude and long-range phase coherence set in simultaneously at the CDW transition temperature Tcdw. Here we investigate, using photoemission, X-ray scattering and scanning tunnelling microscopy, the canonical CDW compound 2H-NbSe2 intercalated with Mn and Co, and show that the conventional view is untenable. We find that, either at high temperature or at large intercalation, CDW order becomes short-ranged with a well-defined amplitude, which has impacts on the electronic dispersion, giving rise to an energy gap. The phase transition at Tcdw marks the onset of long-range order with global phase coherence, leading to sharp electronic excitations. Our observations emphasize the importance of phase fluctuations in strongly coupled CDW systems and provide insights into the significance of phase incoherence in ‘pseudogap’ states.

Tunneling spectroscopy measurements are reported on single crystals of Bi 2 Sr 2 CaCu 2 O 81d using vacuum tunneling and point-contact methods. A reproducible dip feature in the tunneling conductance is found near jeV j 2D, observed for both voltage polarities in the best resolved spectra. With overdoping the position of the dip continues to scale with D, and its magnitude decreases as D decreases. These results indicate that the dip feature arises from a strong-coupling effect whereby the quasiparticle lifetime is decreased at a characteristic energy of ϳ2D, consistent with an electron-electron pairing interaction. [S0031-9007(97)04908-9] PACS numbers: 74.50. + r, 74.62.Dh, 74.72.Hs A provocative feature that has commonly been observed in the superconductor-insulator-normal (SIN) metal tunneling conductances of Bi 2 Sr 2 CaCu 2 O 81d (Bi2212) is a dip at jeV j ϳ 2D that is very large for voltage polarities which correspond to the removal of quasiparticles from the superconductor [1][2][3]. This feature has generated much interest due to its similarity to a dip found in the angle-resolved photoemission (ARPES) [4,5] spectra of Bi2212 and to strong-coupling effects in general [6]. But in contrast to the tunneling phonon structures observed in strong-coupled, low-T c superconductors [6], the dip is often highly asymmetric with bias voltage. For voltages corresponding to electron injection, the dip has appeared as a shoulder in early point-contact tunneling (PCT) measurements [1] and is scarcely observable in some scanning tunneling microscope (STM) measurements [2]. We report here that the dip feature is indeed observed for both bias voltage polarities in the best resolved SIN spectra obtained from both STM and PCT methods. This points toward a strong-coupling interpretation. As T c and D are reduced by overdoping (the latter from 37 to 15 meV in this study), the dip location continues to scale with D and its magnitude is reduced. The coupling to D is quite unusual and suggests that the dip arises from a pairing interaction that is purely electronic so that the superconducting gap feeds back into the excitations which mediate the pairing. This is also consistent with recent interpretations of the dip in ARPES spectra as arising from the coupling of quasiparticles to collective excitations [7,8].SIN tunneling spectroscopy is a unique probe of high temperature superconductors (HTS) in that it can, in principle, reveal the quasiparticle excitation spectrum directly with an energy resolution better than 1 meV. In conventional, s-wave superconductors, strong-coupling effects due to quasiparticle emission of bosons of frequency v produce a dip feature [6] in the tunneling conductance, dI͞dV , near a voltage, eV D 1 "v. More precisely, it is the maximum negative slope if dI͞dV vs V that pinpoints the boson frequency. In d-wave superconductors, which have gap nodes, quasiparticle decay processes which turn on at some threshold energy (e.g., 2D) display a dip in the density of states (DOS) near that energy [9], unshifted...